THE GROWING IMPORTANCE OF THE PLASTID-LIKE DNAS OF THE APICOMPLEXA

Organisms in the phylum Apicomplexa possess, in addition to their mito chondrial genome, an extrachromosomal DNA that possesses significant s imilarities with the extrachromosomal genomes of plastids. To date, th e majority of data on these plastid-like DNAs have been obtained from the human malarial organism, Plasmodium falciparum. In common with pla stid DNAs, the plastid-like DNA of P. falciparum possesses genes for D NA-dependent RNA polymerase subunits beta and beta' and for organellar -like large- and small-subunit ribosomal RNAs. Both the polymerase sub mit and ribosomal RNA gene sequences share a number of features with t hose from plastid DNAs. In addition, the ribosomal RNA genes are organ ised in an inverted repeat arrangement, reminiscent of plastid DNAs. A dditional molecular features shared between the 2 genomes are discusse d. Plastid-like DNAs have also been identified in other Plasmodium spe cies as well as Toxoplasma gondii, Eimeria tenella, Babesia bovis and a number of Sarcocystis species. A cryptic organelle often observed in apicomplexans has been proposed as the organelle that harbours the pl astid-like DNAs, but conclusive evidence for this has not yet been obt ained. Although approximately 1/2 of the plastid-like DNA of P. falcip arum has been sequenced to date, no function has yet been ascribed to this DNA or its putative organelle Phylogenetic inferences based on se quence data from this DNA have indicated an evolutionary origin from p hotosynthetic organisms, but the true provenance of the plastid-like D NAs remains to be determined. Because of the specific nature of the pl astid-like DNAs, they may prove useful as effective targets for chemot herapeutics. Copyright (C) 1996 Australian Society for Parasitology. P ublished by Elsevier Science Ltd.